Since the advent of the tantrum capacitor in the early 1950's, much effort has been expended to develop an effective process to coat solid tantalum capacitor anodes with electrically conductive manganese dioxide. The "solid" tantalum capacitor anodes are porous and are typically prepared using powder metallurgy techniques, e.g. sintering, and then anodization. These porous anodes are then coated, both internally (within the pores) and externally, with electrically conductive manganese dioxide. The manganese dioxide coating acts as the cathode or negative capacitor plate and facilitates the flaw healing or isolation process characteristic of solid tantalum electrolytic capacitors via thermal reduction of the manganese dioxide in contact with flaw sites to a lower, less electrically conductive oxide.
The manganese dioxide coating is typically formed by dipping the anodized tantalum anodes in an aqueous manganese nitrate solution and then placing the manganese nitrate impregnated anodes in an oven at a temperature usually between 200.degree. C. and 400.degree. C. for a sufficient time to pyrolyze the manganese nitrate to manganese dioxide. In actual practice, multiple impregnation and pyrolysis cycles are usually employed, as are various concentrations of the manganese nitrate solution.
Dense, physically strong and highly electrically conductive manganese dioxide coatings are associated with superior device performance, particularly with low device ESR (equivalent series resistance). In attempts to obtain these desirable properties, capacitor manufacturers have varied the number of impregnation pyrolysis cycles, the manganese nitrate concentration, the oven temperature, and the composition of the oven atmosphere. However, none of these methods have been completely successful in obtaining a manganese dioxide coating of superior performance, particularly when producing tantalum capacitors on a large scale.
It was also discovered that the introduction of steam results in the production of more physically dense and electrically conductive manganese dioxide coatings than coatings produced in the absence of added steam. U.S. Pat. No. 3,337,429, for instance, describes injecting steam into the oven. The steam provides water vapor in addition to any water vapor produced by evaporation of the water component of the manganese nitrate solution.
Processes for providing uniform pyrolytic coatings of manganese dioxide on tantalum anodes have proven to be complex and do not always produce the desired results (i.e. dense, conductive coatings). Furthermore, processes which proved effective on a laboratory scale have been very difficult to scale-up to production scale. For example, it has proven difficult to adapt to variations in anode size or oven loading--that is the number of devices pyrolyzed at one time.
Peter H. Klose of Kemet Electronics provided one of the first detailed studies of the pyrolysis of manganese nitrate solutions and the manganese dioxide produced in the reaction. His findings, published in the Journal of The Electrochemical Society, Vol. 117, No. 7, July 1970, Pages 854-858, describe the variable specific gravity (from 2.5 to 4.7 gm/cm.sup.3) and electrical resistivity (from 0.1 to 0.0028 Ohm.cm) of manganese dioxide produced by pyrolysis of manganese nitrate under a variety of conditions. The most important finding appears to be that the highest density, most electrically conductive manganese dioxide is produced under conditions wherein the decomposition gases are confined in close proximity to the pyrolyzing manganese nitrate, preferably under a very slight pressure above atmospheric.
Nishino et al., in U.S. Pat. Nos. 4,038,159 and 4,042,420, have expanded upon Klose's work by using small, positive pressure ovens to produce dense, highly conductive manganese dioxide coatings on tantalum capacitor anodes. According to Nishino et al. these ovens act to confine the decomposition gases (i.e. H.sub.2 O, NO.sub.2, and NH.sub.3 if ammonia is added to the manganese nitrate) to a relatively small volume of space surrounding the anodes and dense, smooth, uniform manganese dioxide layers are produced. See also "Electrical and Physical Properties of MnO.sub.2 Layer for the High Performance Tantalum Solid Electrolytic Capacitor"(second Manganese Dioxide Symposium, Tokyo, 1980, Proceedings published 1981 by the Electrochemical Society)
Aronson et al., in U.S. Pat. No. 4,164,455, goes one step further by not only using a small volume pyrolysis oven, but also by injecting nitrogen dioxide (NO.sub.2) as well as steam into the oven chamber. More specifically, at least 10% by volume of nitrogen dioxide mixed with steam and/or inert gas is injected into the pyrolysis oven at temperatures between 170.degree. C. and 250.degree.. Inert gases include air and nitrogen.
A method to treat manganese nitrate coatings in an oven to produce manganese dioxide coated tantalum anodes having low ESR values for all of the anodes in an oven is desirable.
It is an object of the present invention to provide a tantalum capacitor impregnation method which results in a manganese dioxide coating having a high conductivity.
It is a further object of the invention to provide a method of producing tantalum capacitors exhibiting low ESR.
It is a still further object of the invention to provide a method of producing tantalum capacitors in which all capacitors within an oven have low ESR values.